It has previously been proposed to provide hearing aids in form of small cylindrical elements which include a sound duct structure individually fitted to be introduced into the ear canal of a user, hereinafter, for short, the fitted element. The cylindrical structure, in which the fitted element terminates, is closed off at the far side by a cover which is movable with respect to the remainder of the structure, for example rotatable, to form an ON/OFF switch and a volume control for the hearing aid. This structure has an advantage that only the fitted element must be individually made for the user, the remainder of the housing being of standard construction. Thus, contrary to structures which require individual fitting of the entire hearing aid housing,it is only necessary to individually fit the fitted element. It has been found that the cylindrical housing structure, due to its shape, and depending on the shape of the ear of the user, may project undesirably from the outer ear of the user and may, for some users, press against folds or other parts of the outer ear of the user.